1. Field of the Invention
The present invention relates to a method of making a mask pattern for a mask used in manufacture of semiconductor integrated circuits and so on, an apparatus for making a mask pattern and an apparatus for making such a mask.
2. Description of the Related Art
Lithography techniques have been utilized in manufacturing semiconductor integrated circuits and so on, including the step of printing a mask pattern formed on a mask onto a wafer through the use of an exposure system such as a stepper (a step-and-repeat apparatus). It is desirable to increase depth of focus as much as possible for achieving accuracy in printing a mask pattern. The following methods are known for increasing depth of focus.
For example, a method of increasing a value of .sigma. of an exposure system and a method of oblique incidence illumination such as zone illumination are known for increasing depth of focus. Such methods are effective for a pattern 121, as shown in FIG. 1, having dense traces on a mask 110. However, with such methods an increase in depth of focus is not expected for a sparse (non-dense) pattern 131 as shown in FIG. 2.
A table in FIG. 3 shows depths of focus obtained with a dense mask pattern and a sparse mask pattern under different conditions of secondary light sources wherein a KrF excimer laser stepper is used as an exposure system. Depths of focus are obtained with secondary light sources of flat illumination where values of .sigma. are 0.55 and 0.80, respectively, and quadrupole illumination as oblique incidence illumination. As a dense pattern, the pattern 121 shown in FIG. 1 is used, made up of lines and spaces wherein trace width is 0.20 .mu.m. As a sparse pattern, the pattern 131 shown in FIG. 2 is used, made up of single trace without any other neighboring trace wherein trace width is 0.20 .mu.m.
As shown in FIG. 3, in the case of dense pattern, the depth of focus changes with the secondary light sources of the exposure system. The depth of focus increases with an increase in the value of .sigma. of flat illumination. Furthermore, the depth of focus is greater with oblique incidence illumination.
In the case of sparse pattern in contrast, the depth of focus does not change with the secondary light sources. Furthermore, the depth of focus is lower under any of the conditions compared to the dense pattern. It is thus impossible to increase depth of focus with changing an illumination condition for the sparse pattern. However, a method is known for increasing depth of focus with a sparse pattern through forming a pseudo dense pattern. The method utilizes the fact that depth of focus is increased with a dense pattern compared to a sparse pattern as described above.
FIG. 4 is a top view of a sparse mask pattern wherein a pseudo dense pattern is formed through the use of techniques of related art. In this example a sparse original pattern 141 of width w1 is formed on the mask 110. An auxiliary pattern 142 of width w2 is formed on both sides of the original pattern 141 at intervals `d`. A pseudo dense pattern is thereby formed. Width w2 is required to be lower than the resolution limit of an exposure system used in a printing process so that the auxiliary pattern 142 is not printed on a wafer. The depth of focus of the sparse original pattern is thus increased without printing the auxiliary pattern 142 on the wafer.
FIG. 5 is a plot for showing a relationship between width w2 and resolution dimension of the auxiliary pattern 142 (a graph 151) and a relationship between width w2 and depth of focus of the sparse original pattern 141 (a graph 152) with regard to the pattern with the pseudo dense pattern as described above. The results shown are obtained through light intensity calculation wherein width w1 of the sparse original pattern 141 is 0.20 .mu.m and interval `d` between the original pattern 141 and the auxiliary pattern 142 is 0.20 .mu.m.
As shown in the plot, the closer width w2 of the auxiliary pattern 142 to width w1 of the sparse original pattern 141, the more the depth of focus of the pattern 141 is increased.
However, a theory of auxiliary pattern formation method is not thoroughly established in related art. For example, if an original pattern is a simple repeated pattern, auxiliary pattern formation is relatively easy. However, if an original pattern is a non-repetitive and complex one such as a circuit pattern of semiconductor integrated circuit, formation of auxiliary pattern is complicated as well.
Auxiliary pattern formation has been empirically performed since a method thereof is not theoretically established. It therefore takes a long time for fabricating an auxiliary pattern all over the region on a mask, for example. Consequently auxiliary pattern formation is practically impossible in some cases.